Waste heat recovery devices improve the thermal efficiency of power generators and industrial heating furnaces. Substantial gains in the efficiency of energy usage can be realized if the energy in exhaust gases of such equipment can be used to preheat combustion air, preheat process feedstock or generate steam. One such device to utilize waste heat is the recuperator. A recuperator is a direct transfer type of heat exchanger where two fluids, either gaseous or liquid, are separated by a barrier through which heat flows. The fluids flow simultaneously and remain unmixed. There are no moving parts in the recuperator. Metals, because of their high heat conductivity, are a preferred material of construction provided that the waste heat temperature does not exceed 1600.degree. F. (871.degree. C.).
For a recuperator to provide long service life, conservative designs are required which adequately allow for the principal failure mechanisms. The principal failure mechanisms of metallic recuperators include:
(a) excessive stresses due to differential thermal expansion resulting from temperature gradients, thermal cycling and variable heat flow; PA0 (b) thermal and low cycle fatigue; PA0 (c) creep; and PA0 (d) high temperature gaseous corrosion.
Many early recuperator designs did not take thermal expansion into account. This caused early failure due to excessive stresses created by the failure to allow for thermal expansion. However, as recuperator designs have been improved, the nature of the failure appears to have shifted away from thermally induced stresses and towards thermal fatigue and high temperature gaseous corrosion.
Because recuperators operate, at least in part, above 1000.degree. F. (538.degree. C.), recuperator alloys are subject to carbide and sigma phase precipitation with resulting reductions in ductility and resistance to crack propagation. Further, since sigma and carbides contain large amounts chromium, their formation will deplete chromium from the matrix and thereby accelerate high temperature gaseous corrosion.
Thermal fatigue is the result of repeated plastic deformation caused by a series of thermally induced expansions and contractions. Uniform metal temperature will, of course, minimize thermal fatigue. High thermal conductivity in the metal will minimize, but not eliminate, any existing thermal gradient. Resistance to thermal fatigue can also be enhanced by improving a material's stress rupture strength which is an objective of this invention.
High temperature gaseous corrosion will depend upon the nature of the fluid stream. Where the recuperator is used to preheat combustion air, one side of the barrier metal is subject to oxidation and the other side is subject to the corrosion of the products of combustion. Oxidation, carburization and sulfidation can result from the products of combustion. Nickel-iron-chromium base alloys containing 30-80% Ni, 1.5-50% Fe, 12-30% Cr, 0-10% Mo, 0-15% Co, 0-5% Cb+Ta, plus minor amounts of Al, Si, Cu, Ti, Mn and C, are gererally and adequately resistant to high temperature gaseous corrosion. Non-limiting examples would be for instance, INCONEL alloys 601, 617, 625, INCOLOY alloy 800, etc. (INCOLOY and INCONEL are trademarks of the Inco family of companies.) Preferably, alloys containing 50-75% Ni, 1.5-20% Fe, 14-25% Cr, 0-10% Mo, 0-15% Co, 0-5% Cb+Ta plus minor amounts of Al, Si, Cu, Ti, Mn and C, combine excellent high temperature gaseous corrosion resistance with high strength and thermal ccnductivity and low coefficients of expansion, which minimize thermal stresses due to temperature gradients.
For example, the high thermal conductivities of INCONEL alloys 617 and 625 are 94 (13.5) and 68 (9.8) BTU inch/ft.sup.2 -hr..degree.F. (watt/m-.degree.K.) respectively. The low coefficients of expansion of these two alloys are 7.8.times.10.sup.-6 (1.40.times.10.sup.-5) and 7.7.times.10.sup.-6 (1.34.times.10.sup.-5) in/in-.degree.F. (mm/mm-.degree.K.).
These alloys possess an additional attribute which is a subject of this invention. These alloys can be cold worked and partially annealed to achieve an enhanced stress rupture strength which can be utilized without loss of this enhanced strength in recuperators operating at 600.degree.-1500.degree. F. (316.degree.-816.degree. C.). This additional strength aids resistance to thermal and low cycle fatigue, creep and crack propagation.
It is apparent that the combination of properties required for maintenance--free operation of a recuperator is restrictive. The material of construction must be intrinsically corrosion resistant, possess favorable heat transfer and expansion characteristics and have adequate strength and strength retention at the maximum use temperature. If the strength and strength retention is high, the wall thickness of the barrier may be minimized. This will enhance transfer of heat thus increasing overall thermal efficiency of the recuperator or, alternatively, if the heat transfer is adequate, permit reduction in the amount of material used in constructing the recuperator.
Unfortunately, conventional methods of manufacturing suitable alloy forms such as plate, sheet, strip, rod and bar do not result in products having the optimum physical and chemical characteristics. Conventional cold working of these alloy types result in a product generally too stiff and too low in ductility to be of use in recuperators even though they may have the appropriate tensile strength.
It should be clear that a method of manufacturing alloy forms possessing both the desired physical and chemical characteristics for use in very demanding environments is necessary.